The invention relates to an apparatus and method for detecting gas bubbles in water. The invention also relates to a handling apparatus designed for the recovery, launching and storage of an oceanographic device with a thin, non-self supporting "skin" or fairing.
It has been known for some that the underwater natural gas deposits leak small amounts of natural gas into the overlying water, the natural gas rising to the surface in the form of small bubbles; thus, it is desirable to be able to detect rising columns of such bubbles in water in order to locate sites which are likely to contain gas deposits. Some of these columns of bubbles can be detected visually on the surface as the bubbles break, but such visual observations are effectively confined to very calm conditions since even small waves on the surface render such visual observations effectively impossible. The rising columns of bubbles have also been detected by downward-looking active sonar apparatus. (The terms "active sonar" and "passive sonar" are used herein with their conventional meaning in the art, an active sonar apparatus being one which itself radiates fluid wave energy into the surrounding water and detects echoes caused by this fluid wave energy being reflected back from objects in the water, while a passive sonar system does not itself radiate fluid wave energy but merely listens for any noises present in the water).
A typical prior art active sonar system capable of detecting bubbles of gas or liquid rising through water is described in U.S. Pat. No. 4,001,764 issued Jan. 4, 1977 to Holland et al. This prior art apparatus comprises a submersible, torpedo-shaped device which radiates a sonar beam downwardly and which is towed by a boat, this boat containing recording and other apparatus for interpretation of the sonar signals. The apparatus is intended for use in detecting leaks from submerged pipelines and the method used is a comparative one; the device is towed along the pipeline when it is known to have no leaks; and, after a leak develops, the device is towed along the pipeline and the two acoustical profiles thus generated compared.
Although in theory the Holland apparatus can be used to detect columns of bubbles rising from underwater natural gas deposits, its usefulness for this purpose is seriously impaired by the fact that the sonar beam is downward looking. Thus, only a narrow path directly below the towing boat is scanned at any one time, and the shallower the water in which the device is used the narrower the track scanned. Because of the very large areas of ocean bottom which have to be surveyed for natural gas deposits, it is customary in underwater survey work to survey with a spacing of about one-half mile between adjacent tracks, and the narrowness of the area scanned with the Holland apparatus would mean that deposits falling between tracks would be missed. If the spacing between adjacent tracks is made sufficiently small to ensure that the Holland apparatus does not miss any bubble columns, the areas of ocean bottom surveyed during a working day becomes too small to be practicable. Obviously, any sonar survey system utilizing a downward looking beam will suffer from the same problem.
A further problem with using the Holland apparatus is that, as already mentioned, it relies upon comparing acoustical profiles obtained when no bubbles are rising from the ocean floor and when bubbles are rising. If the apparatus is used to survey previously unchartered areas of ocean bottom for rising bubble columns, it is of course impossible to produce the "no-leak" acoustical profile required for comparison purposes.
It might at first appear that the problem of limited surveying rate in the Holland and similar apparatus could be overcome by directing the sonar beam transversely of the apparatus instead of vertically downwardly. However, the Holland apparatus operates with a high-frequency beam (such beams typically have frequencies of around 60 kHz) and it has hitherto been necessary to use such high frequencies in detecting gas bubbles. Elementary wave theory shows that waves, including sound waves, will only be reflected by objects having dimensions greater than one-half the wave length of the wave. Since the rising gas bubbles from natural gas deposits are known to be only a few centimeters in diameter, while the velocity of sound in water is around 1,500 meters per second, those skilled in the art have hitherto believed that the use of high frequency sonar was imperative for detection of the small bubbles from natural gas deposits. However, high frequency radiated acoustical energy is much more rapidly attenuated in sea water than is low frequency acoustical energy, and for this reason even a side scanning high-frequency sonar apparatus capable of detecting rising columns of gas bubbles would have such a limited range that it would still not be practicable for surveying large areas of ocean bottom. Furthermore, when using such a side scanning apparatus considerable difficulty would be encountered in distinguishing rising columns of bubbles from other reflections due to fish or other objects in the water. A column of bubbles can be spotted relatively easily on a downward looking sonar trace because the spread of the bubbles through the water produces (on the conventional type of sonar plot which effectively plots depth of reflections against distance traveled by the apparatus) a characteristic trace showing reflections over a large vertical interval, as opposed to the sharp reflections at a particular depth caused by fish or other objects. On the other hand, a side scanning sonar beam would intersect the narrow vertical column of bubbles transversely and it is not immediately apparent how the reflection produced by this transverse intersection with a rising bubble column could be distinguished by the echo produced by any other small object.
There would also be problems involved in handling a side-scanning sonar apparatus. Conventional fluid wave energy transducers are of such a shape that they cannot be satisfactorily towed by themselves. Accordingly, the actual transducers which effect radiation and detection of fluid wave energy must be mounted within a streamlined casing or fairing to give good hydrodynamic qualities to the apparatus. The transducers will only radiate and detect fluid wave energy efficiently while they are immersed in water, so the fairing must flood while the apparatus is submerged in water. Also, to prevent excessive dissipation of fluid wave energy as the sonar beam passes through the fairing that part of the fairing through which the sonar beam passes must be made so thin that it is relatively fragile and is easily damaged during launching or retrieval of the apparatus. Providing an appropriately thin fairing does not present much difficulty in a downward looking sonar apparatus since the sonar beam only passes through the lower part of the fairing and thus a fairing can be used which has a relatively thin and fragile lower section and a relatively thick, rigid and strong upper section, all the manipulation and the handling of the fairing during launching and recovery being effected by manipulating the rigid upper part of the fairing. However, in a side scanning sonar apparatus, the sides of the fairing must be thin (in practice, a side scanning apparatus will always use two side scanning beams on opposed sides of the apparatus) and the handling of the resultant fragile fairing poses grave difficuties.
It will thus be seen that there is a need for a method and apparatus for rapidly scanning large areas of water to detect rising columns of gas bubbles therein. It will also be seen that there is a need for an apparatus capable of handling a sonar apparatus which uses a very fragile fairing. This invention seeks to meet both these needs.